I never got a chance to play one, but I'm still curious to hear from anybody that actually got play one a significant amount of time.
From a long-time player PoV, they're fun - moderately tough, high-DPR beatsticks, evocative of the 1e fighter - for as long as that can be fun for you. For me the nostalgia wore off in about an hour.
I also played a Slayer as an Archer, which was clearly an off-label use (Power Strike only works in melee, so it's throwing away even encounter powers in return for doubling up on DEX bonus to damage, still quite effective). It was fun because it was a reprise of a 3.5 Barbarian/Thief character, a primary archer who would rage to kick it when forced into melee, this worked out similarly, when 'forced' into melee he'd throw down Power Attack. Not quite as cool, but I'd developed a whole personality for the character that made it fun.
From a system perspective, like much of Essentials, The Slayer (like other martial E-classes) is more problematic. Really, Essentials and the mass of errata that accompanied it could be seen as ill-advised complication (ironically in the name of simplification) or even the intentional sabotage of an otherwise pretty decent game. In the case of the Slayer, specifically, it deviates from AEDU (adding to the overall complexity of the game, including making day length matter to class balance), deviates from Role support (being a striker with defender AC, hps & surges), can abuse the many minor options that let you buff basic attacks (it's at-wills & encounters all augment basic attacks), and, though it is presented as a fighter build or sub-class, introduces at-will and encounter powers that cannot be used by previous fighter builds (a first) and, similarly, cannot avail itself of a lot of extant fighter material (contrast that to the Mage, which can take any extant wizard power and provides a bunch of new & improved wizard powers other wizard types can use).
They SEEM like a pretty solid option.
They are a solid enough striker in Heroic (it's hard to do striker too badly, it seems, even the Vampire is a pretty good striker - all 4 possible Vampire characters you can play), also 'solid' in the sense of being tougher than typical for non-primal strikers. But they lack options, resources and tactical flexibility compared to 4e classes, and even most other E-classes, limiting their long-term appeal. But they do peter out pretty quickly - Essentials, in general, like 3e-and-earlier & 5e today, seems to have been designed without an expectation of higher level (11+) seeing a lot of use.